You will need a lathe and machining experience. Installing a factory barrel on factory guns with the factory contour just requires checking headspace, but most people who want a barrel replaced are looking for something better than what the factory can offer.
The headspace is a measurement from the bolt face to a point on the cartridge. This point is different on different case styles. Too little and the bolt wont close. Too much and the cartridges sloshes around in the chamber. Adjusting headspace has to do with moving the chamber and/or the barrel to fall with in a window measured by the go and no-go gauges.
AR-15 bolts headspace in the barrel extension. This is set when the extension is installed on the barrel so the armorer doesnt (much) have to worry about it, and can change barrels on uppers without too much thought.
Barrel threading is just like any other threading operation. Turning down a barrel to a different contour can be fun. Machining for gunsmithing is easy to pick up if you have a good teacher.
The best way to learn is to take classes or intern with a master smith. The NRA offers summer courses in machining and rebarreling at a few of the gunsmithing schools. It is probably possible to buy a $3k Grizzly and teach yourself, but you will save money in broken reamers, gauges, and barrel blanks by having someone teach you.
TSJC (the best school) classes are listed here: http://nra.trinidadstate.edu/
Other NRA classes are listed here: http://www.nrahq.org/education/gunsmithing.asp